1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a method and apparatus for measuring a quantity of residual toner, wherein the residual toner can be estimated by considering the length of time the associated toner cartridge has been in use. Aspects of the present invention provide a means wherein a user can be informed of the quantity of residual toner.
2. Description of the Related Art
In general, an image forming apparatus, such as a laser printer, comprises a paper-feeding cassette, a toner cartridge containing developer, a print engine for forming and transferring a visible image to a paper fed from the paper-feeding cassette, a fixing unit for fixing the transferred visible image to the paper, and a controller for controlling respective components of the image forming apparatus. The controller also calculates the used or residual toner quantity which is used as developer at the time of printing.
The toner contained in the toner cartridge is developed on a photosensitive medium formed with an electrostatic latent image through a predetermined process, and the toner developed on the photosensitive medium is transferred to a transfer medium by a transfer device. After the toner has been transferred, the photosensitive medium is electrically charged after undergoing a cleaning process, then an electrostatic latent image is formed again on the photosensitive medium, then toner is developed again on the photosensitive medium, and these steps are repeated. The toner transferred to a sheet of paper in this manner is subjected to a fixing process by adequate heat and pressure while being passed through a fixing unit, and one print operation is completed as the paper is discharged.
A conventional method for measuring the quantity of residual toner in such an image forming apparatus is to provide a toner detecting sensor having a light emitting unit and a light receiving unit on a toner cartridge of the print engine. The quantity of toner is detected and, if desired, the user is informed. However, such a method has a problem in that the toner detection sensor and related detection circuit are additional items required, which increases manufacturing costs.
Another conventional method for providing information regarding the quantity of residual toner is to counter the number of pages of papers printed since a new toner cartridge has been changed. This method judges whether toner is exhausted when a quantity of papers exceeding a predetermined number of pages have been printed, and to inform the user that the toner is exhausted. However, such a method calculates the quantity of residual toner on the basis of an average quantity of toner required for printing one page, which is estimated on the basis of about 4% to 5% coverage. The “4% to 5% coverage” means that the quantity of toner printed on the paper is 4% to 5%, 100% occurring when toner is printed on the entire page. However, since a printed image is generally composed of various patterns—including dot, line, character, text, diagram, picture or the like—the quantity of toner actually used on a given sheet of paper may vary. Therefore, a toner cartridge may not actually be used with as many pages of paper as anticipated.
A recent conventional solution has been to apply a dot counting method to measure the quantity of residual toner. In the dot counting method, each time a page is printed the number of dots per page is counted. When multiple pages are printed, the number of dots per page is counted and accumulated for the totality of pages printed.
The number of printable pages (Pr) is calculated by subtracting the number of printed pages per print job (Pj) from the quantity of residual toner stored in the residual toner data memory, in other words, the number of residual printable pages (Pr−m). This calculation e may be expressed by an equation as follows:Pr=Pr−m−Pj, andPj=Qj÷Qr (for each environment).
In the equations above, Qj is the quantity of consumed toner per job and Qr is the quantity of consumed toner per reference page, in which the latter is set on the basis of 5% coverage for A4 size page.
Since the quantity of consumed toner per job (Qj) indicates the quantity of transferred toner (Qt) plus the quantity of produced waste toner (Qw), the number of printable pages may be expressed as follows:Pr=Pr−m−{(Qt+Qw)/Qr}.
The quantity of transferred toner (Qt) is calculated by adding the values obtained by multiplying the number of dots per each page (Ndot), the quantity of transferred toner per dot (qdot) and a weighting factor (solid/text/gray), and is expressed as follows:Qt=Σ(x·Ndot·qdot).
In the equation above, x is a weighting factor that changes depending on various image information of a print data, such as dot, line, character, text, diagram, picture, or the like.
As can be seen from the equations above, the quantity of residual toner, that is the number of residual printable pages (Pr), can be calculated only when the quantity of toner per dot (qdot) consumed is calculated. Thus, the quantity of residual toner can be calculated only when the quantity of transferred toner (Qt) and the quantity of waste toner (Qw) have been calculated.
However, the conventional method for measuring quantity of residual toner is calculated by using calculated values rather than by accurately measuring the quantity of transferred and waste toner. Accordingly, as the quantity of toner used or the length of time that the toner is used increases, an offset is created that represents deviation from the actual value of residual toner. In general, once a toner cartridge is installed for printing, toner stress, deterioration and wear of components occur as time goes by. Toner stress increases as time goes by so that the toner can no longer sufficiently exhibit the performance originally possessed by fresh toner. This results in poor image quality. Furthermore, since image developing efficiency and electrified characteristic (Q/M) of the toner are also changed, the quantity of produced waste toner is also changed. Moreover, the performance of essential components such as an image developing roller and feeding roller of the toner cartridge, a photosensitive medium, and a charging roller, also deteriorate, thereby worsening the above-mentioned problems. In addition, because various toner parameters depend on density of print data or printing environment, an offset deviation from actual value occurs.
Due to the problems identified above, there will be an error between the measured value and actual value as to the quantity of residual toner. If the error is large, a user cannot be informed of the correct time for changing the toner cartridge because the information related to the quantity of residual toner, life span of one or more components, quantity of waste toner, or the like, is incorrect. Ultimately, image quality cannot be maintained.